Surgeons frequently form holes in bones. Such holes are, in many instances, subsequently tapped to allow the insertion of metal or plastic screws. Screws are used to fasten devices which repair degenerative, congenital or trauma related defects.
Defects in the hole resulting from drilling or tapping of the hole, or from insertion of the screw commonly result. Screw threads or screw tips protrude from the bone due to, for example, a defect in cortical integrity from improper placement or preparation of the hole.
A protrusion of the threads from the hole, or a tip of the screw protruding from the hole, or other defects in the hole, may result in the screw failing to hold properly. Further such defects may cause complications to the patient resulting from the surgery. For example, if a hole is drilled and tapped for the placement of pedicle screws during lumbosacral stabilization, cauda equina or nerve root injury may result. Neurological deficit to the patient may therefore occur due to the incorrect placement or tapping of the pedicle hole.
Techniques which are commonly used for determination of screw hole placement include radiographs, fluoroscopy or visual inspection of the wall. These means are not reliable.
Palpation of the prepared screw hole has also been employed. One method of palpating the hole involves the use of a wire probe by which the surgeon feels the circumference of the hole for any area which that is not encompassed with bone. This method is inaccurate, and can actually cause neurological deficit from contact with the probe. Where a tapped hole is present, the reliability of palpation further diminishes.
Devices are available which detect the differences in conductivity between bone and body tissue. If the bone is not providing complete "insulation", then an indication is provided that there is a defect in the integrity of the screw hole. This method is probably more accurate than the methods discussed above, but is relatively expensive, and does not give an indication of the size or location of the defect.
A very simple test for checking screw hole integrity involves placing water within the screw hole. If there is a defect in the screw hole, the water will run out. Through the use of water containing a dye, a better indication of the location of the screw hole is indicated. This method can be used in most applications, but gives little indication of the size of the defect.